A scary thought occurred to me. What if Trump leveraged his considerable media exposure to rally Republicans to register as Democrats and vote for him in the 2020 primaries? After all, a sitting president usually runs unopposed in his own primaries so he could sacrifice the votes. If he pulled it off, he would be the top of the ticket for both parties and would be almost assured a second term.

Maybe I'm missing the obvious but...democrats (specifically, the DNC) doesn't support Trump. Primaries are run by the parties themselves, so aren't in any shape or form binding in terms of the actual vote.
– user1530Mar 13 '17 at 4:02

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You're assuming the media is friendly to him. They manifestly are not. They were in the tank for Clinton in 2016 and only supported him because he brought in ratings, and they thought he was the best way to ensure a Clinton victory (Pied Piper candidate). It's highly implausible that any of the parties necessary for such a move would be coopted.
– J DoeMar 13 '17 at 19:26

@J_Doe "You're assuming the media is friendly to him. They manifestly are not." Advertisers used to say: "Talk well or bad about him, but talk about him". What brings the votes is the attention, not the tone. Furthermore having appeared for so many years on a TV show helped a lot. Overall his success in the election owes a lot to the media system.
– FluidCodeMar 7 at 16:11

Why do you find it scary? Wouldn't be better to see being done openly what is usually done in disguise with candidates designed to win the primaries and lose the elections?
– FluidCodeMar 7 at 16:16

It is very unlikely that Trump would be able to mobilize that many Republicans to switch parties.

remember that 15% of the delegates of the Democratic National Convention are superdelegates: They are elected officials and high-ranking members of the Democratic Party who can vote however they want. It is very, very unlikely that they would vote for a Republican incumbent. That means Trump would need to mobilize considerably more support than his strongest Democratic contender. When he already has that much support, he would be able to win the re-election without resorting to such tricks.

Most states do not allow electoral fusion. That means Republican-Trump and Democrat-Trump would be treated as separate voting options. That means the Trump-vote would be split and a 3rd-party candidate (who would likely be considered far more relevant than usual in that case) would have a far easier time to win.

Does splitting the vote for a candidate in the New York system have any practical implications? I don't think so. If a candidate gets 30 percent of the vote on the party X ticket and 30 percent on the party Y ticket, that candidate still gets 60 percent of the vote and wins, even if opposed by only one person who ran on only one ticket and received 40 percent of the votes on that ticket. Also, each state makes its own rules. Are you sure this is possible in every state, or even a majority of states?
– phoogMar 13 '17 at 0:21

Thanks for the link. The question reminds me of Ed Koch, who ran for re-election as mayor of New York on both the Republican and Democratic tickets (and no doubt some others as well). But in that race, as in most, votes were ascribed to the original candidate as outlined in my previous comment. In presidential races, votes are actually for convention delegates (primaries) or electors (general election), so, depending on the state's system, splitting the vote could have a practical effect, if electors R and D get 30% of the vote each while elector I gets 40%.
– phoogMar 13 '17 at 13:30

A decent centrist 3rd party candidate would also be likely to benefit hugely from faithless electors, I would assume.
– ScottMay 11 '17 at 23:11

@phoog Koch is a nice example. In NY state people can't cross vote, so only registered republicans could vote in the republican primary. Koch won it in 1981 rather easily. (A couple years after the Republicans were calling that election a mistake and committed to never again and rebuilding the base. 1981 was a strange time with NYC coming out of bankruptcy and Koch being popular and conservative for a democrat. Nice example though, but not possible for someone like Trump.
– userLTKNov 6 '17 at 7:20

A major problem with this is the assumption that Trump would not face any challenges from other Republicans. While Trump won the 2016 nomination, he did so not having won a majority of primary votes (just shy of 45% of all Republican primary votes were cast for Trump). Running simultaneously as a Democrat would split his resources (he would be fighting a political war on two fronts). A politically weak Trump (assuming he doesn't improve his approval ratings by 2020) might lose enough delegates to lose one or both nominations. It's also likely that some of the stronger candidates from 2016 (Marco Rubio, or Ted Cruz) still have enough of their political machines to restart in 2020.

A controversial aspect of the Democratic caucuses is the lack of a secret ballot. That means people will have to live with the vote they cast in front of their friends and neighbors for the next four years. And the faint of heart may be susceptible to pressure from more vociferous contemporaries.
Thus, a candidate who wins the first round of a caucus is not home safe. They can still end up losing if their supporters fail to win over backers of candidates eliminated for not passing the threshold and instead see that support go elsewhere -- one reason why the organization and training of precinct captains is so crucial for campaigns.
It's also why the second choices of voters for a presidential candidate are so important in the Democratic caucuses.

Trump is deeply unpopular with many Democrats. It would be hard to see lots of Democrats publicly voting for him as their party's nominee, especially after what would have likely been 3 years of criticizing him.